JSFiddle - React, Tailwind, and code Playground

HTML

<script
  src="https://code.jquery.com/jquery-1.12.4.js"
  integrity="sha256-Qw82+bXyGq6MydymqBxNPYTaUXXq7c8v3CwiYwLLNXU="
  crossorigin="anonymous"></script>
  <link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
  <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js"></script>
  <script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
   <link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">

   
  <label>date_debut</label><input type="text"name="date_debut" id="datepicker">
</br>
<label>date fin</label><input type="text"name="date_fin"id="datepicker2">
</br>
<label>nombre jour</label><input type="text"name="nombre_jour" id="result">

<button id="button">Calcul</button>

</button>


</br>

CSS

label{
  width:120px;
}

JavaScript

$('#datepicker, #datepicker2').datepicker();

$('#button').click(function () {
    var start = moment($('#datepicker').val()),
        end = moment($('#datepicker2').val());

    
    var diffInDays = end.diff(start,"days");
  
		if(isNaN(diffInDays) != true)
    	$("#result").val(diffInDays);

});